1. Field of the Invention
The present invention generally relates to digital data files stored in a memory and, more specifically, files with non-divisible sections, that is, having their physical recording in a memory area performed by sections of fixed length or linearly.
The present invention more specifically relates to the management of the accesses to files according to rights (of writing, reading, deleting, copying, etc.) granted to users or to applications. Such rights are a function of keys or codes which are assigned to the different users or to the different functions or applications.
The present invention more specifically applies to the field of smart cards or the like for which the file system processes non-divisible sections of fixed size.
2. Discussion of the Related Art
Currently, a key conditioning rights of access to a file having as a parameter to have to be written in non-divisible fashion by sections or linearly, can only be assigned to the entire file or to a section of this file. The distribution of the rights of access must be compatible with the granularity (minimum size) of each section read from or written into the memory.
This constraint is little disturbing in applications where the memory is almost unlimited (computer hard disk, for example). However, in applications of smart card type, or more generally as soon as the memory size is critical with respect to the information volume to be stored, the granularity of the file section often curbs the increase of the number of applications that the system can process.
For example, in the case of a contactless smart card (transponder) applied to the management of transport documents, the management systems (size of the exploited information, number of different categories of access rights, etc.) often differ from one network to another (from one town to another, from one operator to another, from one country to another, etc.). For a same smart card to be able to operate on several networks, it must comprise as many files as there are networks, each file being dedicated to a network.
Such a limitation reduces the possibilities of sharing a smart card between several applications.
The same problems are posed, even for variable-size sections, if the file writing is linear, that is, not dividable at will. For simplification, reference will be made hereafter to the granularity of data in a memory to designate the minimum size of the undividable sections which can be read from it or written into it. However, all that will be described in relation with granularity applies to linear-writing variable-size sections.
A solution would be to decrease the granularity of the memory sections (in the extreme, down to a bit) according to the minimum size of the information to which specific access rights are desired to be granted. Such a theoretical solution however cannot be envisaged in practice, for several reasons.
First, this solution would require modifying the memory management devices (direct memory access controller or DMA), or even the memory structure (granularity of the input-output amplifiers).
Further, the access right management would require assigning a key to each element of minimum granularity. This would multiply the number of keys and would require, only to manage the rights, a memory of a size greater than that of the memory containing the information.
Further, the user or the application supposed to have access would have to have as many keys as there are sections. Such a system would accordingly rapidly become unmanageable.